Functional assay for 5-HT2A, histamine H1 or adrenergic alpha 1B receptors

ABSTRACT

The present invention provides novel functional assay for 5-HT 2A , histamine H1 or adrenergic alpha 1b receptors, by measuring intracellular cyclic adenosine monophosphate (cAMP) levels utilizing reporter gene driven cell based assay. The novel assay provides both binding affinity as well as mode of action of compounds in a single set. The novel assay of the invention is useful in identification of compounds acting through 5-HT 2A , histamine H1 or adrenergic alpha 1b receptors. Furthermore, the assay offers utility in categorizing compounds in to agonist, partial agonist, inverse agonist and antagonist classes. The novel assay can be scaled up to any high throughput format.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Application No. PCT/IN2010/000209, filed on Mar. 31, 2010, which in turn claims priority to Indian Patent Application No. 3194/CHE/2009, filed Dec. 29, 2009, the contents of which are both hereby incorporated by reference.

FIELD OF INVENTION

The present invention describes establishment of a cell based functional assay for 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors, by determining the level of intracellular cAMP levels utilizing reporter gene driven cell based assay. The novel assay described in the current invention, combines features of binding and functional mode in a single type of experimental set up which offers both binding affinity as well as mode of action of compounds interacting with any 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors in a single set. The novel assay of the invention is useful in identification of compounds acting through 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors and their further categorization based on the mode of action. The assay may find high utility in screening of new chemical entities for identification of novel drugs acting through 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors. Moreover, the novel assay can also be utilized in exploring the mode of action of established and known drugs and assignment of their pharmacological properties.

BACKGROUND OF THE INVENTION

GPCRs also known as seven-transmembrane domain receptors comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and ultimately cellular responses. GPCRs are involved in various physiological functions as well as many diseases, and are also the target of around half of all modem medicinal drugs. There are two principal signal transduction pathways involving the GPCRs: cAMP signal pathway and Phosphatidylinositol signal pathway.

The cytoplasmic tail region of GPCRs interacts to one of the three main classes of G proteins. G proteins are composed of a common βγ subunit and a specific α subunit. The role of the α subunit is well known for translating the extracellular cues to intracellular responses. G proteins are mainly classified into three categories based on the nature of their subunits. G proteins containing Gαs or Gαi subunits enhance or reduce the cAMP level respectively upon receptor stimulation through adenylate cyclase enzyme. In contrast, G proteins comprising Gαq subunit mobilize intracellular calcium ions upon activation of the receptor through a membrane bound phospholipase C enzyme. In recent years, Gβγ subunits turned out to be more than a silent partner. They too transmit the message through activation of ERK-MAPK pathways.

The molecular diversity of GPCR-mediated signal transduction pathway complicates the configuration of a common functional assay. The development of high through-put functional assays for GPCRs would greatly enhance the ability to discover and develop novel agonists and antagonists to this important superfamily of pharmaceutical targets. One approach for developing a high through-put functional GPCR assay is the use of reporter gene assays. Reporter gene constructs couple transcriptional enhancers that are regulated by various intracellular second messengers with appropriate promoter and reporter gene elements to produce a surrogate signal transduction system responsive to signaling pathways activated by various hormone receptors (Deschamps, Science, 1985 230:1174-7; Montminy, Proc. Nail. Acad Sci USA, 1986 83:6682-6686; Angel, Cell, 1987, 49:729-39; Fisch, Mol. Cell. Biol, 1989 9:1327-31). With the appropriate choice of transcriptional enhancers, promoters, and reporter genes, non-radiometric functional assays have been configured for Gαs coupled GPCRs (Konig, Mol. Cell. Neurosciences, 1991, 2:331-337; Chen, Anal. Biochemistry, 1995, 226: 349-354) and Gαq coupled GPCRs (Weyer, Receptor and Channels, 1993 1:193-200) that are amenable to high through-put screening technology.

Earlier, some assays have been developed for the identification of GPCRs. However, some disadvantages are associated with the traditional radioligand binding assay and FLIPR based functional assays. In traditional binding assay a) hazardous radioactive ligands are used; b) the mode of action of a molecule needs to be further investigated using a separate radioactive or cell based functional assay. In the FLIPR based functional assay (European patent application: EP1310800), fluorescent counts are measured within 5 seconds of compound injection which may not be sufficient to bring the compound in equilibrium with the receptor. As a result of shorter incubation of compounds in FLIPR assay, important compounds may be missed out or artefactual values may be generated.

In order to overcome the above mentioned disadvantages, we have developed novel functional assay for 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors based on measurement of intracellular cAMP levels by measuring reporter gene activity:

-   -   a) which eliminates the usage of hazardous radioactive ligands.         Thus an environmental friendly approach can be implemented to         identify novel compounds.     -   b) mode of action (agonist or antagonist) as well as binding         affinity of the ligand (pK_(b) or pEC50) are derived from the         same experiment.     -   c) compounds are incubated for few hours to bring them in         equilibrium with the receptor. Longer incubation period in         reporter gene format is required to have a sustained activation         of the receptor leading to the synthesis of cAMP and induction         of the reporter gene.

Thus, the reporter gene based assay may be more in line with the physiological conditions wherein the drug is allowed to interact with the target receptor in the body.

SUMMARY OF INVENTION

In one aspect, the present invention provides a novel functional assay method for identification of compounds acting through GPCRs, which comprises:

-   -   a) transfecting chinese hamster ovary (CHO) cells with specific         5-HT_(2A), histamine H1 of adrenergic alpha 1b receptors         constructs along with express of reporter gene using         Lipofectamine® 1-Propanaminium, N-(3-[(4[(3-aminopropyl) amino)         butyl) amino) propyl)         amino)-3-oxopropyl)-N,N-dimethyl-2,3-bis(((9z)-1-oxo-9-octadecenyl)oxy)-,         2,2,2-trifluoroacetate (1:1), mixt. with 1-(((2-aminoethoxy)         hydroxyphosphinyl)oxy)methyl)-1,2-ethanediyl         di-(9z)-9-octadecenoate and cultured in suitable medium         containing selection antibiotics;

In one aspect, the present invention provides a novel functional assay method for identification of compounds acting through 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors, which comprises:

-   -   b) picking up individual colonies exhibiting maximum luciferase         activity with both forskolin as well as the agonist and         analyzing the expression of said 5-HT_(2A), histamine H1 or         adrenergic alpha 1b receptors;     -   c) plating recombinant CHO cells in a microtitre plate and         cultured;     -   d) incubating said recomninant cells with increasing         concentration of compounds at 37° C. in CO₂ incubator for         evaluation of compounds in antagonist mode and agonist mode of         5-HT_(2A), histamine H1 or adrenergic alpha 1b receptors;     -   e) after incubation of the cells was over, removing the medium         and washing the cells with buffer and lysed in the lysis buffer;     -   f) measuring CRE-Luc reporter gene activity in individual wells.

In another aspect, total RNA isolated from each cell line was used in the cDNA synthesis.

In further aspect, radioligand binding and competition assays were performed with membranes prepared from the recombinant CHO cell lines.

In yet another aspect, mode of action (agonist or antagonist) as well as binding affinity of the ligand (pK_(b) or pEC50) is derived from the same experiment.

BRIEF DESCRIPTION OF DIAGRAMS

FIG. 1 shows expression of the RNA in recombinant CHO cells.

FIG. 2 shows stimulation of luciferase reporter gene in CHO cells expressing human 5-HT_(2A), adrenergic Alpha_(1b) or Histamine H₁ receptors.

FIG. 3 shows ligands stimulate the specific receptor in a dose dependent manner.

FIG. 4 shows blockade of reporter gene expression by antagonists.

FIG. 5 shows dose response graph of serotonin on CHO-5HT_(2A) cells (in the presence of 10, 100 and 1000 nM Ketanserin), dose response graph of epinephrine on CHO-α_(1b) (in the presence of 10, 100 and 1000 nM Ketanserin) and dose response graph of histamine on CHO-H₁ cells (in the presence 10, 100 or 1000 nM Cetirizine). The agonist concentration used was same as in FIG. 3.

FIG. 6 shows graphical representation of method for screening of 5-HT_(2A), histamine H1 or adrenergic alpha 1b receptor compounds.

DETAILED DESCRIPTION OF INVENTION

Luciferin, T4 DNA Ligase, high fidelity Taq polymerase, superscript reverse transcriptade, mammalian vector pcDNA3.1, CRE-Luc reporter gene, cell culture media, sera, radioligands Ketanserin Hydrochloride [ethylene-³H] 60-90 Ci/mmol, Prazosin [7-methoxy-³H] 70-87 Ci/mmole, Pyrilamine [pyridinyl 5-³H], (Mepyramine) 20-30 Ci/mmol, scintillation proximity assay beads, Human 5-HT_(2A) cDNA clone, Adrenergic alpha1b cDNA clone, all other DNA restriction enzymes, modification enzymes, all other reagents and common chemicals were purchased from well known suppliers.

Human 5-HT_(2A) cDNA clone was amplified by polymerase chain reaction (PCR) using gene specific primers. Adrenergic alpha_(1b) cDNA clone was amplified by PCR using gene specific primers. Human histamine H₁ cDNA was generated by reverse transcription using total RNA isolated from HepG2, 1MR32, HEK293 and CaCo2 cell lines and gene specific reverse primers. The cDNA was amplified by PCR using gene specific primers using high fidelity Taq DNA polymerase. Amplified DNA was cloned in to mammalian expression vector pcDNA 3.1. The authenticity of the cloned genes was determined by restriction analysis and nucleotide sequencing.

EXAMPLE 1 Expression of the RNA in Recombinant CHO Cells

Total RNA from recombinant or control CHO cells were isolated using TRI reagent (Sigma) as recommended. The quality of each RNA sample was analyzed by agarose gel electrophoresis. Total RNA from each cell line was used in the cDNA synthesis by reverse transcription using Superscript Reverse Transcriptase and gene specific reverse primers for 5-HT_(2A), Alpha_(1b), H₁ and β-actin genes. PCR was performed on each cDNA sample using gene specific forward and reverse primers. Samples were separated on 1% agarose gel and visualized after Ethidium bromide staining.

As evident from FIG. 1, human 5-HT_(2A) is exclusively expressed in CHO cells transfected with human 5-HT_(2A) gene (CHO-5HT_(2A)) and not in control and other transfected cells. Similarly human histamine H₁ or adrenergic alpha_(1b) mRNA are exclusively expressed in CHO cells transfected with human H₁ or α_(1b) cDNA (CHO-H₁ or CHO-α_(1b)) respectively. β-actin mRNA was also reverse transcribed in the same set of assays using a gene specific reverse primer. The cDNA from each of the cell line was amplified using β-actin specific forward and reverse primers. Presence of a band corresponding to β-actin fragment in FIG. 1 demonstrated the presence and good quality of the RNA in each preparation.

EXAMPLE 2 Stimulation of Luciferase Reporter Gene in CHO Cells

Each recombinant CHO cell line was plated in 96 well white with clear bottom plates. The ligands as indicated were added to a final concentration of 10 μM. The luciferase activity was measured in individual wells using luciferin substrate in Perkin Elmer luminometer. The basal luciferase activity for each cell line was assigned an arbitrary value of 1. Fold stimulation with each ligand was determined in relation to the basal luciferase activity.

The assay was also utilized to investigate the specificity of various ligands and authenticity of the generated cell lines in this assay. As evident from FIG. 2, serotonin at a concentration of 10 μM demonstrated about nine fold induction of luciferase activity in CHO-5HT_(2A) cells. However, histamine and epinephrine at the same concentration did not show a significant induction of luciferase activity in CHO-5HT_(2A) cells. Similarly, epinephrine at a concentration of 10 μM induced the expression of luciferase activity to about six fold in CHO-α_(1b) cells. As expected, CHO-α_(1b) cells did not respond to the treatment with 10 μM serotonin or histamine (FIG. 2). CHO-H₁ cells exhibited about three fold enhanced expression of luciferase enzyme with 10 μM histamine. Epinephrine and serotonin (10 μM each) did not show any effect on the level of luciferase activity in CHO-H₁ cells. A strong induction in luciferase activity was observed in all the three recombinant cell lines upon treatment with forskolin.

EXAMPLE 3 Ligands Stimulate the Specific Receptor in a Dose Dependent Manner

To further confirm the single dose effect of specific agonists on the recombinant CHO cell lines, a dose response effect with different agonists was measured. Each recombinant cell line was treated with individual agonists from 0.1 nM to 10,000 nM and luciferase activity was measured by using luciferin substrate in Victor Light Luminometer from Perkin Elmer. The agonist stimulated luciferase activity in the absence of a compound was assigned a value of 100% while basal luciferase activity was assigned a value of 0%. Rest of the luminescent values obtained for compounds at various doses were calculated with reference to stimulated and basal luciferase activities. Data was analyzed using Graphpad software.

Serotonin showed a specific dose response effect on the expression of luciferase activity in CHO-5HT_(2A) cells (FIG. 3). A pEC₅₀ value of 6.6 for serotonin in the assay was determined (Table 1). Epinephrine and histamine did not show the induction of luciferase activity in CHO-5HT_(2A) cells up to the highest dose tested (FIG. 3). 5-HT_(2A) receptor is not reported to couple to Gαs containing G protein or stimulate cAMP production under any experimental conditions reported so far. 5-HT_(2A) receptor is well established to couple to Gαq containing G protein (Raymond J R. Mukhin Y V, Gelasco A, Turner I, Collinsworth G, Gettys T W, Grewal J S, Garnovskaya M N: Multiplicity of mechanisms of serotonin receptor signal transduction. Pharmacol Ther 2001, 92: 179-2 12). Some reports also suggest inhibition of cAMP accumulation upon activation of 5-HT_(2A) receptor (Garnovskaya M N, Nebigil C G, Arthur J M, Spurney R F, Raymond J R: 5-hydroxytryptamine 2A receptors expressed in rat renal mesangial cells inhibit cyclic AMP accumulation. Mol Pharmacol 1995, 48: 230-237). A range of pK_(i) values from 8.4 to 6.0 are reported for serotonin in 5-HT_(2A) radioligand binding assays in IUPHAR database. The pEC₅₀ value determined in the current investigations falls in the middle of the reported range.

CHO-α_(1b) cells showed a robust dose response with increasing concentrations of epinephrine from 0.1 to 10000 nM as evident from the level of luciferase activity observed (FIG. 3). The treatment of recombinant cells with epinephrine provided a pEC₅₀ value of 7.3 (Table 1). pEC₅₀ values of 7.3 and 6.6 for calcium mobilization and cAMP accumulation respectively are reported for CHO cells expressing human adrenergic alpha1b receptor upon treatment with norepinephrine (Horie K, Itoh H, Tsugimoto G: Hamster α_(1b)-adrenergic receptor directly activates Gs in the transfected chinese hamster ovary cells. Mol Pharmacol 1995, 48: 392-400), (Gallego M, Setien R, Puebla L, Boyano-Adanez MdC, Arilla E, Casis O: α₁-adrenoceptors stimulate a Gαs protein and reduce the transient outward K⁺ current via a cAMP/PKA-mediated pathway in the rat heart. Am J Physiol Cell Physiol 2004, 288: C577-C585). CHO-α_(1b) cells did not respond to histamine or serotonin treatment as evident from basal level of luciferase activity observed at the highest concentration of the ligand tested (FIG. 3).

Following on the same pattern, CHO-H₁ cells were treated with increasing concentrations (1 to 100,000 nM) of histamine. A dose proportionate increase in luciferase activity was observed in CHO-H₁ cells treated with histamine but not with serotonin or epinephrine. Histamine showed a pEC₅₀ value of 6.0 in this assay. Moniri et. al. reported a pEC₅₀ value of 5.8 in a cAMP accumulation assay in CHO cells expressing human histamine H₁ receptor upon treatment with histamine (Moniri N H, Covington-Strachan D, Booth R G: Ligand-directed functional heterogeneity of histamine H1 receptors: novel dual function ligands selectively activate and block H1-mediated phospholipase C and adenylyl cyclase signaling. J Pharmacol Exp Ther 2004; 311: 274-281) Overall, the values reported in the present assay are well in agreement with the published values generated using different strategies for three receptors under investigation.

TABLE 1 pEC₅₀ values Compounds CHO-5HT_(2A) CHO-α_(1b) CHO—H₁ Serotonin 6.6 ± 0.1 <5 <5 Epinephrine <5 7.0 ± 0.3 <5 Histamine <5 <5 6.0 ± 0.1

EXAMPLE 4 Blockade of Reporter Gene Expression by Antagonists

Once we observed an induction of reporter gene activity in all recombinant cell lines evaluated in a agonist and dose dependent manner, it was of interest to determine whether the activity was blocked by various known antagonists. A number of compounds already demonstrated to antagonize some or all of the receptors under investigation were selected for the current study. Vehicle or selected compounds (10 μM concentrations) were incubated along with 10 μM of specific agonist with the cells and luciferase activity was measured by Victor Light Luminometer from Perkin Elmer. A detailed evaluation of various compounds in specific cell lines is presented in FIG. 4.

EXAMPLE 5 Dose Response Study

Ketanserin, mianserin, olanzapine, clozapine and chlorpromazine (each at 10 μM concentrations) fully antagonized the serotonin induced and 5-HT_(2A) mediated induction of luciferase activity in CHO-5HT_(2A) cells. While haloperidol exhibited a limited antagonism, cetirizine did not show any impact on serotonin induced luciferase activity in CHO-5HT_(2A) cells. Risperidone demonstrated an inverse agonism on 5-HT_(2A) receptor and brought down the luciferase activity to less than vehicle control. A treatment of CHO-5HT_(2A) cells with increasing concentrations of serotonin (0.1 to 10,000 nM) in the presence of 0, 10, 100 and 1000 nM ketanserin resulted in a significant right shift in the graph (FIG. 5). The rightward shift in the graph in FIG. 5 was dependent on the concentration of ketanserin used, with maximum shift observed with 1000 nM dose. To better understand the effect of these compounds on serotonin induced luciferase activity in CHO-5HT_(2A) cells, a dose response study was performed. The cells were incubated with the increasing concentrations of above compounds (0.1 to 10,000 nM) along with 10 μM serotonin and luciferase activity was measured. Table 2 provides a pIC₅₀ as well as pK_(b) value for each compound tested in all three cell lines. As evident from Table 2, majority of the compounds demonstrated a potent antagonism to serotonin induced luciferase activity in CHO-5HT_(2A) cells with a pIC₅₀ value of close to or less than 7.0 with the exception of cetirizine and haloperidol. Cetirizine and haloperidol exhibited none or partial antagonism to serotonin induced luciferase activity in CHO-2A cells. Serotonin completely displaced ketanserin in CHO-5HT_(2A) cells. Similarly, ketanserin fully displaced serotonin when assayed in the same cell line. All these observations support a competitive and reversible binding of ligands to CHO-5HT_(2A) cells.

The effect of same set of compounds, tested in CHO-5HT_(2A) cells, was investigated in CHO-α_(1b) cells. Haloperidol, clozapine, chlorpromazine and risperidone (at a concentration of 10 μM) completely blocked the epinephrine induced luciferase activity in the recombinant cells (FIG. 4). In comparison to the above compounds, ketanserin, olanzapine and mianserin showed a limited efficacy, cetirizine did not show any effect on the epinephrine induced luciferase activity in CHO-α_(1b) cells. Chlorpromazine and risperidone demonstrated a potent antagonism to epinephrine induced luciferase activity in CHO-α_(1b) with pIC₅₀ values of 6.9 and 7.1 respectively (FIG. 4, Table 2). Rest of the compounds showed a pIC₅₀ value of close to or less than 6.0. As expected, cetirizine did not show any antagonism of epinephrine induced luciferase activity. We studied the effect of various concentrations of ketanserin on the level of luciferase activity in CHO-α_(1b) cells upon treatment with increasing concentrations of epinephrine. Selection of ketanserin for this evaluation was guided by the observation that it had limited impact on blocking the epinephrine induced luciferase activity in CHO-α_(1b) cells. As evident from FIG. 5, ketanserin did induce a rightward shift in the graph as compared to control epinephrine treated samples. The extent of rightward shift in curves was dependent on the concentration of ketanserin used with the maximum shift at 1000 nM dose. However, the effect of ketanserin on CHO-α_(1b) cells was milder than on CHO-5HT_(2A) cells which are well in agreement with the fact that it has a higher affinity for human 5-HT_(2A) receptor as compared to alpha 1b receptor. The assay supported a reversible mode of interaction of epinephrine as well as kentanserin to CHO-α_(1b) cells.

CHO-H₁ cells were evaluated for the blockade of histamine induced luciferase activity by a defined set of compounds. As evident from FIGS. 2 and 4, CHO-H₁ cells exhibited about four fold stimulation of luciferase activity with 10 μM histamine which correlated well with a direct cAMP measurement in CHO cells expressing human histamine H₁ receptor reported. Histamine induced luciferase activity was completely blocked by H₁ specific antagonist cetirizine (FIG. 4). In addition, all the tested compounds with the exception of haloperidol (at a concentration of 10 μM) blocked the histamine induced luciferase activity to the basal level. Effect of different concentrations of cetirizine on the histamine induced luciferase activity in CHO-H₁ cells was investigated. Cetirizine treatment resulted in a rightward shift in the curves plotted with an increasing concentration of histamine (FIG. 5). Cetirizine also demonstrated an inverse agonist property as evident from FIG. 4. Most of the compounds evaluated showed a potent antagonism of histamine induced luciferase activity in CHO-H₁ cells in a dose dependent manner. Mianserin demonstrated to be the most potent compound in the assay with a pIC50 value of 8.1. Cetirizine showed a pIC₅₀ value of 6.3.

The assay apart from demonstrating various mode of action of a compound to GPCRs can also be utilized to characterize orthosteric or allosteric mode of action of a compound. Furthermore, the reporter gene based functional assay can identify whether a compound interacts with the receptor in a reversible or irreversible manner.

EXAMPLE 6

Once the reporter gene assay was established and detailed pIC₅₀ and pK_(b) values were determined for a number of compounds, it was of interest to measure their binding affinity for the same set of receptors. Although the binding parameters for majority of these compounds are already reported, it was important to evaluate them in parallel with the reporter gene based functional assays.

TABLE 2 pIC₅₀ and pK_(b) values obtained from functional assays CHO-5HT_(2A) CHO-α_(1b) CHO-H₁ Compounds pIC₅₀ pK_(b) pIC₅₀ pK_(b) pIC₅₀ pK_(b) Ketanserin 7.1 ± 0.2 9.0 ± 0.2 5.4 ± 0.2 7.7 ± 0.2 5.9 ± 0.1 7.4 ± 0.2 Mianserin 7.1 ± 0.2  8.9 ± 0.04 5.0 ± 0.3 7.4 ± 0.3  8.1 ± 0.04 10.1 ± 0.01 Cetirizine <5 <5 <5 <5 6.3 ± 0.2 8.4 ± 0.2 Olanzapine 7.5 ± 0.3 9.3 ± 0.1 5.7 ± 0.1 7.9 ± 0.2 7.99 ± 0.3   10 ± 0.3 Haloperidol 5.8 ± 0.4 7.5 ± 0.1 6.2 ± 0.1 8.6 ± 0.1 5.4 ± 0.2 7.4 ± 0.4 Clozapine 6.5 ± 0.4 8.3 ± 0.2 6.0 ± 0.2 8.3 ± 0.2 7.8 ± 0.3 9.8 ± 0.3 Chlorpromazine 6.9 ± 0.3 8.6 ± 0.1  6.9 ± 0.05  9.3 ± 0.05 6.75 ± 0.06 8.8 ± 0.2 Risperidone 8.5 ± 0.3 10.3 ± 0.2  7.1 ± 0.1 9.5 ± 0.1  5.9 ± 0.06 7.9 ± 0.1

Radioligand binding and competition assays were performed with membranes prepared from the recombinant CHO cell lines as described in following three references:

-   1. Nelson D L, Lucaites V L, Wainscott D B, Glennon R A: Comparison     of hallucinogenic phenylisopropylamine binding affinities at cloned     human 5-HT_(2A), 5-HT_(2B) and 5-HT_(2C) receptors.     Naunyn-Schmiedeberg's Arch Pharmacol 1999, 359: 1-6. -   2. Yoshio R, Taniguchi T, Itoh H, Muramatsu I: Affinity of serotonin     receptor antagonists and agonists to recombinant and native     α1-adrenoreceptor subtypes. Jpn J Pharmacol 2001, 86: 189-195. -   3. Ratnala V R P, Swarts H G P, VanOostrum J, Leurs R, DeGroot H J     M, Bakker R A, DeGrip W J: Large scale overproduction, functional     purification and ligand affinities of his-tagged human histamine H₁     receptor. Eur J Biochem 2004, 271:263 6-2646.

All the assays were converted to scintillation proximity assay (SPA) based format. For competition binding assays, 0.1 to 10,000 nM of each compound was incubated with a fixed concentration of the specific radioligands, membrane and SPA beads. For adrenergic alpha_(1b) receptor, 4.0 nM of Prazosin [7-methoxy-³H] was incubated with recombinant membrane and Polylysine (PLL) coated Yitrium silicate (Ysi) SPA beads for three hours at ambient temperature. For, histamine H_(I) binding assay, 1 nM of Pyrilamine [pyridinyl 5-³H] was incubated with the membrane and Wheat Germ Agglutinin (WGA) coated Polyvinyl Toluene (Pvt) SPA beads for two hours at the ambient temperature. For 5-HT_(2A) binding assay, 3.1 nM Ketanserin Hydrochloride [ethylene-³H] was incubated with the membrane and WGA coated Ysi SPA beads in dark for four hours. After incubation was over, radioactivity was measured in MICROBETA® plate reader (Perkin Elmer). Total binding was determined in the absence of any ligands whereas non-specific binding was determined to be counts obtained in the presence of excess amount of specific ligands. Specific activity was calculated from the differences between total and non-specific counts.

Radioligand binding assay using SPA beads were performed to determine the pK_(i) values for specific compounds. The binding experiment was performed thrice and average values are presented in Table 3. As evident, all the compounds showed a pK_(i) value which well correlates with the already reported values. These compounds also demonstrated target selectivity as reported earlier (www.iuphar-db.org/GPCR/ReceptorFamiliesForward). Serotonin exhibited a pK_(i) value of 6.8 to 5-HT_(2A) receptor but did not show any binding to alpha_(1b) and H₁ receptors. Similarly, epinephrine showed strong binding to alpha 1b receptor with a pK_(a) value of 6.4 but a weak affinity for 5-HT_(2A) and no binding to H₁ receptors. The pK_(i) value determined is well in agreement with the published value. Histamine did not show any binding to either 5-HT_(2A) or alpha_(1b) receptors while giving a pK_(i) value of 5.9 with H₁ receptor which is again well in agreement with previously reported values. Ceterizine demonstrated a selective binding to human H₁ receptor with a pK_(b) value of 7.7 and no affinity to 5-HT_(2A) or H₁ receptors. Clozapine, mianserin, ketanserin and olanzapine showed varying degree of binding to all the above receptors (Table 3).

TABLE 3 pK_(i) values determined from radioligand binding assays Compounds/ Targets 5-HT_(2A) α_(1b) H₁ Serotonin  6.8 ± 0.02 <5 <5 Epinephrine 5.1 ± 0.1 6.4 ± 0.1 <5 Histamine <5 <5  5.9 ± 0.01 Clozapine 7.8 ± 0.1 7.2 ± 0.2 8.8 ± 0.1 Mianserin 8.2 ± 0.2 6.9 ± 0.1 9.7 ± 0.1 Ketanserin 8.9 ± 0.1 7.2 ± 0.1  7.7 ± 0.03 Olanzapine 7.5 ± 0.1 6.5 ± 0.1 8.6 ± 0.1 Cetirizine <5 <5 7.7 ± 0.1

EXAMPLE 7

pK_(i) and pK_(b) are derived parameters from pIC₅₀ value which are determined by binding assays and functional assays respectively. As the pIC₅₀ value for the same ligand and receptor combination may vary depending on the amount of radioligand used in the binding assay or amount of agonist used in the functional assay, derived pK_(i) and pK_(b) values demonstrate a constant parameter for a specific compound. Thus, we compared the pK_(i) and pK_(b) values for specific compounds derived from radioligand binding assay or cell based function assay. For an agonist, pEC₅₀ value derived from the functional assay is compared with the pK_(i) value determined from the binding assay. As evident from Table 4, majority of the compounds showed a good correlation between pK_(b) or pEC₅₀ values derived from the functional assay and pK_(i) values generated from the radioligand binding assay.

TABLE 4 pEC₅₀, pK_(i) and pK_(b) value comparison 5-HT_(2A) α_(1b) H₁ Compounds/ pEC₅₀ or pEC₅₀ or pEC₅₀ Targets pK_(b) pK_(i) pK_(b) pK_(i) or pK_(b) pK_(i) Serotonin 6.6 ± 0.1  6.8 ± 0.02 <5 <5 <5 <5 Epinephrine <5 5.1 ± 0.1 7.0 ± 0.3 6.4 ± 0.1 <5 <5 Histamine <5 <5 <5 <5 6.0 ± 0.1  5.9 ± 0.01 Clozapine 8.3 ± 0.2 7.8 ± 0.1 8.3 ± 0.2 7.2 ± 0.2 9.8 ± 0.3 8.8 ± 0.1 Mianserin  8.9 ± 0.04 8.2 ± 0.2 7.4 ± 0.3 6.9 ± 0.1 10.1 ± 0.01 9.7 ± 0.1 Ketanserin 9.0 ± 0.2 8.9 ± 0.1 7.7 ± 0.2 7.2 ± 0.1 7.4 ± 0.2  7.7 ± 0.03 Olanzapine 9.3 ± 0.1 7.5 ± 0.1 7.9 ± 0.2 6.5 ± 0.1  10 ± 0.3 8.6 ± 0.1 Cetirizine <5 <5 <5 <5 8.4 ± 0.2 7.7 ± 0.1 

We claim:
 1. An assay method for screening for antagonists and agonists of 5-HT_(2A) receptor, histamine H1 receptor or adrenergic alpha 1b receptor by a reporter gene based assay in a microtitre plate comprising: a) transfecting chinese hamster ovary (CHO) cells with a specific G-Protein Coupled Receptor (GPCR) coupled to a G protein containing Gαq subunit construct, wherein the specific GPCR is selected from 5-HT_(2A) receptor, Histamine H1 receptor and adrenergic alpha 1b receptor along with excess of cAMP response element luciferase (CRE-luc) reporter gene with 1-Propanaminium, N-(3-[(4[(3-aminopropyl) amino) butyl) amino) propyl) amino)-3-oxopropyl)-N,N-dimethyl-2,3-bis(((9z)-1-oxo-9-octadecenyl)oxy)-, 2,2,2-trifluoroacetate (1:1), mixt. with 1-(((2-aminoethoxy)hydroxyphosphinyl)oxy)methyl)-1,2-ethanediyl di-(9z)-9-octadecenoate and culturing the resultant recombinant CHO cells in a suitable medium containing a selection of suitable antibiotics; b) selecting individual colonies of recombinant CHO cells exhibiting maximum luciferase activity to both forskolin and an agonist of said specific GPCR and analyzing the expression of said specific GPCR; c) plating and culturing the selected recombinant CHO cells in a microtitre plate; d) incubating said cultured, recombinant CHO cells with increasing concentrations of a test compound at 37° C. in CO₂ incubator and evaluating the effect of the test compound as an antagonist or agonist of said specific GPCR; e) after incubation of the cells is over, removing the medium, washing the cells with buffer and lysing the cells with lysis buffer; and f) measuring the CRE-Luc reporter gene activity in individual wells of the microtitre plate.
 2. The method as claimed in claim 1, wherein said luciferase reporter gene constructs is three fold in excess.
 3. The method as claimed in claim 1, wherein said microtitre plate is used for high throughput screening. 